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When an eastern Massachusetts homeowner planned to add two bedrooms to his house, he learned that he would have to upgrade his septic system—to the tune of $75,000 and a 5-foot-high mound system. Today, a patch of evergreens, holly, hostas, and irises creates an inviting landscape where that mound would have been. An advanced biofilter system was installed after the septic tank, with leachate dispersed via drip-irrigation tubing through a landscape of hardy plants specially chosen for their abilities to take up wastewater.
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PHOTO: CAROL STEINFELD |
| A system cleans wastewater in the courtyard of an office building located near Chapel HIll, NC. |
In back of a home on Cape Cod, a sturdy patch of 8-foot-high bamboo in a polyethylene-lined contained garden bed treats the home’s graywater. Blackwater is managed separately with a toilet stool that drains to an aerobic composting toilet unit below the floor. This system option saved the homeowner an estimated $30,000 over a new septic-and-mound system, a common prescription for failing septic systems in poorly draining soils.
Increasingly, landscapes will do double duty as part of wastewater treatment systems that “grow clean water.”
The main drivers: stricter wastewater regulations as well as rising water rates that could make drinking water too costly to use for irrigation.
Demand for ecological wastewater treatment will grow steadily as scientists and engineers find that landscape-based root-zone systems clean wastewater better than the conventional wastewater disposal leachfields, even in colder regions. Studies show 99.9% pathogen destruction in these systems.
At the Root of It All
Some use the term “natural systems” to describe these landscape-based, plant-and-aggregate-based wastewater systems, but this is a bit of a misnomer, as conventional treatment also uses natural biology. The ecological engineering field calls them “constructed ecosystems.”
Some constructed ecosystems are wet, or “saturated,” and some of them are not. As a design option, this depends on the treatment goals, the soils, and the climate.
Treating wastewater involves destroying pathogens, reducing biological oxygen demand (BOD), filtering out particles, reducing or using up nitrogen and phosphorus, and stabilizing or disposing of toxins. All of this happens faster in constructed ecosystems than in conventional treatment systems. In a septic tank, for example, effluent often is not treated; it is just settled. All pathogens, toxins, and nitrogen (mostly from urine) still drain from the tank into the soil, where some transformation occurs. Instead of disposing of the leachate 2–6 feet underground as conventional leachfields do, constructed ecosystems keep effluent in the first 18 inches, the “biological zone.”
The key difference is allowing more oxygen into the system. Oxygen-using (aerobic) bacteria transform and stabilize effluent 10–20 times faster than anaerobic bacteria. The second advantage is plants. They establish the root system, or rhizosplane, where the real transformation occurs. The roots provide a home for beneficial bacteria to transform effluent. In the terminology of wastewater treatment plant operators, the substrate and the roots behave like “fixed-film reactors,” supporting microbes that like to be attached to something while preying on pathogens, turning nutrients into a form that plants can use, and converting nitrogen to gas.
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PHOTO: CAROL STEINFELD |
| A washwater garden in Boston treats and uses up all effluent from a washing machine, helping to reduce the load on the city's wastewater treatment plant as well as to support community landscaping. |
Plants also add air to the system through “capillarity” (think of plant roots and stems as drinking straws). It is a commensal relationship. The pollutants feed the microbial community, and its byproducts are absorbed by the plants. In turn, the plants support the microbial community: Through photosynthesis, their roots receive nutrients—called “exudates”—in the form of complex carbohydrates (mostly sugars) that help maintain the microbes.
As the roots decay, they provide the carbon required by the denitrifying bacteria to convert nitrates into nitrogen gas. (In mechanical treatment systems, sugar is added for this purpose.)
Constructed ecosystems are also self-organizing and adaptive. Plants provide responsive treatment that adapts to changing wastewater strengths and climate changes. Although biological systems work faster in warm environments, they are successful in colder climates too.
By and large, microbes do the work, and microbes are poikilothermic: Their metabolic processes are directly proportional to the temperature.
For every 19 degrees Fahrenheit, their metabolic rate doubles. At 40 degrees Fahrenheit, most microbes are dormant. But wastewater is usually warm, and microbial action generates heat, so treatment may slow during cold seasons but rarely, if ever, stop.
Know the Systems
Generally, a wastewater system starts with a collection system and settling tank, such as a septic tank. If the site requires higher pretreatment before discharge, packaged treatment plants, biofilters, media filters, or treatment systems can be used. These are essentially boxes containing aerators, peat, geotextiles, or complex plastic shapes.
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PHOTO: CAROL STEINFELD |
| A greenhouse encloses a planted evapotranspiration system, which also provides the home's inhabitants with solar heat and extra living area. The health department of Montague, MA, mandated that a home on a marshy lot must use a zero-discharge wastewater system—recommending this one. |
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PHOTO: CAROL STEINFELD |
| A Solar Aquatics/Living Machines sequenced agriculture demonstration system occupies a greenhouse in Burlington, VT. |
In a very environmentally oriented building, the flows may be kept separate: Water used for washing, called “graywater,” may be drained separately from high-nitrogen toilet water, or “blackwater,” so that it can be strategically used.
The discharge then flows to a constructed ecosystem. This is often a bed or series of trenches full of aggregate—such as gravel, peastone, sand, crushed concrete, and even crushed glass and mulch—of various depths and lengths, lined and unlined, with or without outflow, and wet, damp, or dry. And, ideally, planted. On a restricted site, further denitrification and disinfection may be required.
Five Constructed Ecosystems
Perhaps the best-known constructed ecosystem, although its name is often incorrectly used to describe the others, is the constructed wetland.
Typically chosen when there is plentiful land, constructed wetlands are planted beds or trenches filled with coarse media such as gravel that support aquatic vegetation and provide both aerobic and anaerobic conditions. To reduce phosphorus, wetlands require periodic plant harvesting; this should be performed before the onset of summer.
There are two types of constructed wetland: the freewater surface constructed wetland and the subsurface-flow constructed wetland.
In freewater surface constructed wetlands, water flows on top. One can sometimes paddle boats on them but cannot walk on them.
These wetlands are chosen where habitat that attracts wildlife, such as birds and fish, is desired (however, remember that animals can contribute their own e-coli bacteria to the water).
They also provide opportunities for waterfalls and water-flow features, enhancing both evaporation and aeration. Ideally the water will flow fast enough that mosquito larvae cannot take hold in the moving flow; if not, mosquito-eating fish and birds should be present.
With subsurface-flow constructed wetlands, water flows three to eight inches under the surface, so the system can be walked on, driven over (even paved over), and integrated into pathways. These work better than free-flowing wetlands in cold climates as they are insulated from surface air and retain more heat. The sizing is about the same as free-flowing wetlands. The deeper they are, the more anaerobic, which assists with denitrification, the conversion of nitrogen-to-nitrogen gas. Five-foot-deep and deeper subsurface-flow constructed wetlands are used for denitrification. A shallow system is more aerobic, promoting nitrification (which changes ammonia to nitrate—NH3 to NO3—a form of nitrogen accessible to plants) as well as evaporation. However, the shallower the system, the more susceptible it is to cold weather. Depending on the climate, designers might make a hybrid system.
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PHOTO: CAROL STEINFELD |
| Each of the guest houses at Lalati Resort, in Fiji, features a Wastewater Garden growing away all liquid wastewater from the building. A clear Lexan overhang allows sunlight in but keeps precipitation out, so the system's footprint remains small. |
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PHOTO: CAROL STEINFELD |
| Wastewater Gardens also do double-duty as landscaping in cooler climes, where they may be planted with a wide range of hearty grasses and evergreens. |
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PHOTO: CAROL STEINFELD |
| Greenhouse systems also offer their owners the presence of a relaxing, contemplative environment. |
To reduce BOD, aerobic processes work best. The first 12 inches is the zone of aeration, so a system may start at 12-inches deep to reduce BOD and to nitrify, and then head to greater depths of 19 inches to five feet for denitrification.
Planted rock filters are less understood as ecosystems. Unlike their cousins the constructed wetlands, these are more like constructed “damplands” and not always saturated. Common in some southern states such as Arkansas, these systems were designed and tested by NASA. Very flexible, they can be used for both stormwater and wastewater.
Effluent enters the system three to eight inches below the surface. The advantage of the system is that stormwater provides an ebb and flow that both flushes the system and optimizes all of the biological processes by providing a complex and diverse ecology. Because they do not require wetland plants, they are more versatile for landscape features. Rain flows through the rock filter but doesn’t collect and saturate, so non-aquatic plants, including shrubbery and vines, can be used.
Planted evapotranspiration systems, also known as “recirculating wastewater gardens” are designed to use up effluent. Evapotranspiration systems are typically trenches filled with gravel and distribution pipe, and planted with especially thirsty plants, or “phreatophytes.” Treatment is primarily unsaturated and aerobic. These systems are usually chosen to reduce or eliminate the cost of pumping a septic or holding tank on sites where effluent cannot be discharged at all.
Effluent first enters a tank then is pumped into a bed of media such as gravel. Anything not evapotranspired away is drained back to the tank. Because these systems are almost always specified where wastewater must be completely used up and not discharged, they are often lined.
A good lining is a 20- to 40-millimeter, chlorine-free, low-density polyethylene film.
Solar aquatics systems, or “living machines,” are sequences of aquatic tanks (think of aquaria filled with plants) and indoor constructed wetlands that are often enclosed in greenhouses.
They replicate a vertical pond system. Inside the tanks, aerators bubble in oxygen and agitate the mix. At their best, they resemble towers of flowers, foliage, and even trees. They are chosen when advanced treatment is required, such as for reuse in a building. These systems can also process sewage solids or “sludge.”
Methods of distributing the outflow from these systems (if there is any) include:
- Subsurface irrigation: After treatment, effluent is distributed under 6 inches of suitable pervious soil, typically by pressure-dosed, small-diameter perforated pipe or through flexible drip tubing with emitters. Remember that drip irrigation is for dispersal, not treatment, and effluent must be extremely well filtered to prevent clogging of emitters.
- Surface irrigation: Effluent is disinfected and distributed on the surface, typically through conventional sprinklers or other clean-water irrigation systems.
To complete a full, ecologically elegant, onsite water-management strategy, add plumbing for reuse. Treated and disinfected effluent from the constructed ecosystems can be filtered, possibly disinfected, and piped back to the building for use in toilet flushing and make-up water for evaporative cooling systems.
Success Factors
Designers must take into account legally mandated design flows from the building. Precipitation and temperature must be factored in, as well as leaf drop—fallen leaves add carbon to the system—unless leaves are raked off. Many wetlands fail, because the substrate is too small, causing clogging.
Designers are currently using as large as 3/4- to 3-inch stones. The larger the pore spaces, the lower the likelihood of clogging.
Plant Palettes
Plant palettes for constructed ecosystems ideally include facultative plants that do not mind having their roots wet or dry. In wetlands, only obligative plants (roots are always wet) are used. Start with fast-growing plants such as grasses and water hyacinth for a base of workhorse plantings, then accent with showy plants. Look for broad-leafed plants, because the more leaf area there is, the more evapotranspiration takes place. Vines allow for maximum leaf area with minimal footprint.
Select plants that look good and smell good to keep people taking care of them, as well as plants that do not need care, such as bamboo and holly.
For shallow nitrogen-removal systems, clumping bamboos such as phyllostacys aurea, phyllostacys bisetia, and phyllostacys nuda work well. Most of these systems are lined with 20-millimeter polyethylene liners, which have been proven to prevent spreading of bamboo.
If wetlands are planted with reeds, both the foliage and the leaf mass may need to be harvested to remove carbon.
Where phosphorus is an issue, such as where the outfall is surface water, both the leaf mass and the roots should be harvested periodically. For cold-climate systems, select evergreens that are cold tolerant. Hardy evergreens do not go dormant in the winter, so their root zones are active even in the snow.
As in any landscape, 10%–40% of the plants may have to be replaced during the start-up period.
Also, there is a start-up time before optimal treatment is seen: The warmer it is, the sooner it occurs. Constructed ecosystems often self-select what plants work best.
In systems with no outflow that are sheltered from precipitation, and so are not regularly flushed out, salt may build up. This can be flushed out with water, or the system can be planted with halophytes—plants that take up salt—such as mangrove and tamarax (salt cedar).
Who Can Design Them?
In most states, a local permit is required for an onsite wastewater system managing less than 10,000 gallons per day (gpd). (The design flow per bedroom is usually about 110 gpd.) Systems managing more than 10,000 gpd require a federal permit.
Many states certify professional engineers to design smaller systems; however, exceptions may be made for those who can demonstrate the necessary skills. Some states, such as Iowa and Vermont, allow anyone to design systems, as long as the systems comply with state guidelines.
As states regulate for higher treatment of wastewater, constructed ecosystems will become increasingly common treatment modalities. Civil engineers will seek out the services of landscape designers to help them integrate these systems into their clients’ sites. The distinction between landscape and wastewater treatment system will soften, as the ability of soils, substrate, and plants to clean wastewater and runoff is recognized.
As this land-based approach replaces the current plumbing approach to wastewater management, landscapes will be commonly called on to both clean effluents and grow valuable plant products and beauty.
Resources
Some ecological wastewater system consultants:
2020 Engineering, Bellingham, Washington; www.2020engineering.com
Ecological Engineering Group, Concord, Mass.; www.ecological-engineering.com
Living Machines, Inc., Taos, New Mexico; www.livingmachines.com
North American Wetland Engineering, Forest Lake, Minnesota; www.nawe-pa.com
Natural Systems International, Santa Fe, New Mexico; www.natsys-inc.com
Cahill Associates, West Chester, Pennsylvania; www.thcahill.com
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Books to read:
Treatment Wetlands, by Robert Kadlec and Robert Knight; CRCPress.
Small and Decentralized Wastewater Management Systems, by Ronald Crites and George Tchobanoglous; McGraw-Hill.
Growing Clean Water: Nature’s Solution to Water Pollution, by Bill Wolverton; Wolverton Environmental Services.
Wetland Design: Principles and Practices for Landscape Architects and Land Use Planners, by Robert L. France; W.W. Norton.
Constructed Wetlands in the Sustainable Landscape, by Craig Campbell and Michael Ogden; Wiley & Sons.
Start at the Source, by Tom Richmond; Bay Area Stormwater Management Association.
Water Use and Conservation, by Amy Vickers; WaterPlow Press. |
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CAROL STEINFELD is a freelance writer and projects director for Ecowaters projects. DAVID DEL PORTO is principal and senior designer of Ecological Engineering Group.
OW - March/April 2006 |
